Penicillins and cephalosporins are the most frequently and widely used β-lactam antibiotics in the clinic. However, the development of resistance to β-lactam antibiotics by different pathogens has had a damaging effect on maintaining the effective treatment of bacterial infections. (Coleman, K. Expert Opin. Invest. Drugs 1995, 4, 693; Sutherland, R. Infection 1995, 23 (4) 191; Bush, K, Cur. Pharm. Design 1999, 5, 839–845). The most significant known mechanism related to the development of bacterial resistance to the β-lactam antibiotics is the production of class-A, class-B and class-C serine β-lactamases. These enzymes degrade the β-lactam antibiotics, resulting in the loss of antibacterial activity. Class-A enzymes preferentially hydrolyze penicillins where as Class-C lactamases have a substrate profile favoring cephalosporin hydrolysis. (Bush, K.; Jacoby, G. A.; Medeiros, A. A. Antimicrob. Agents Chemother. 1995, 39, 1211). To date over 250 different β-lactamases have been reported (Payne, D. J,: Du, W and Bateson, J. H. Exp. Opin. Invest. Drugs 2000, 247) and there is a need for a new generation of broad spectrum β-lactamase inhibitors. Bacterial resistance to these antibiotics could be greatly reduced by administering the β-lactam antibiotic in combination with a compound which inhibits these enzymes.
The commercially available β-lactamase inhibitors such as clavulanic acid, sulbactam and tazobactam are all effective against class-A producing pathogens. Clavulanic acid is clinically used in combination with amoxicillin and ticarcillin; similarly sulbactam with ampicillin and tazobactam with piperacillin. However, these compounds are ineffective against class C producing organisms. The mechanism of inactivation of class-A β-lactamases (such as PCI and TEM-1) has been elucidated. (Bush, K.; Antimicrob. Agents Chemother. 1993, 37, 851; Yang, Y.; Janota, K.; Tabei, K.; Huang, N.; Seigal, M. M.; Lin, Y. I.; Rasmussen, B. A. and Shlaes, D. M. J. Biol. Chem. 2000, 35, 26674–26682).
In 1981, the Beecham group disclosed 6-alkylidine penems of general structure 1 as inhibitors of β-lactamases. [N. F. Osborne, U.S. Pat. No. 4,485,110 (1984); N. F. Osborne, Eur. Pat. Appl. 81 301683.9.]
R1 and R2 are independently hydrogen or a C1-10 hydrocarbon group or mono heterocyclic, and R3 represents a hydrogen or an organic group. Subsequently, the same group disclosed compounds of the general formula 1, wherein R1 comprises a 1,2,3-triazole moiety. [N. F. Osborne, Eur. Pat. Appl. 84301255.0]. The following year, the same group filed three patent applications of the structure 1, wherein R1 is an optionally substituted six membered or five membered mono aromatic ring system. [N. F. Osborne, Eur. Pat. Appl. 85100520.7; Eur. Pat. Appl. 85100521.5 and Eur. Pat. Appl. 85300456.2]. European patent application No. 86305585.1 discloses the synthesis and the utility of (Z)-6-(1-methyl-1,2,3-triazol-4-ylmethylene)-penem-3-carboxylate 2 as a class-A and class-C β-lactamase inhibitor. 
Eur. Pat. Appl. 86305584.4 disclosed the preparation of compounds of general formula 1, wherein R1=non-aromatic heterocyclic group and a PCT application [N. J. Broom; P. D. Edwards, N. F. Osborne and S. Coulton PCT WO 87/00525] disclosing R1=fused bicyclic hetero-aromatic group was published. Similarly patent applications [N. J. Broom; G. Brooks; S. Coulton, Eur. Pat. Appl. 88311786.3; N. J. Broom; G. Brooks; B. P. Clarke, Eur. Pat. Appl. 88311787.1) disclosed the preparation and use of compounds of general structure 1, wherein R1 is a substituted five membered hetero-aromatic ring. A process for the preparation of compounds of general formula 1 has been disclosed by Coulton, et al. [S. Coulton; J. B. Harbridge; N. F. Osborne and G. Walker Eur. Pat. Appl. No 87300193.7]
In the year 1993, Beecham disclosed [A. V. Stachulski and R. walker, PCT WO 93/03042] the preparation and the use of compounds of general formula 1, in which R1=(C1-6) alkyl and R2=CH2X or COY wherein X=halogen or CONR2.
During the last decade three patents have been filed by Beecham describing compounds of general formula 3. [N. J. Broom; F. P. Harrington, PCT WO 94/10178; K. Coleman; J. E. Neale PCT WO 95/28935; K. Coleman; J. E. Neale PCT WO 95/17184] wherein Ra=hydrogen or an organic group, and Rd and Re may be both hydrogen or one or more substituents replacing hydrogen atoms in the ring system shown below. 